The present invention relates to a method for making a paper based product which incorporates a polymeric latex binder. More specifically, the present invention relates to the use of an emulsion in the manufacture of paper based products which have incorporated a polymeric latex binder. The present invention also relates to the manufactured paper products, which products exhibit excellent physical properties, especially tensile strength.
The papermaking industry as well as other industries have long sought methods for enhancing the strength of products formed from fibrous materials such as, for example, paper and board products formed of cellulose fiber or pulp as a constituent. The problems and limitations presented by inadequate dry strength have been particularly acute in the numerous industries where recycled furnish or fiber mechanically derived from wood is utilized in whole or part. In the papermaking industry for example, recycled cellulose fiber is typically used in the manufacture of newsprint and lightweight coated papers. These recycled fibers, however, are of a generally shorter length than chemically-pulped fibers which in turn provides paper having relatively poor dry-strength properties in comparison to paper manufactured from virgin, chemically pulped fiber. The use of virgin chemically pulped fiber for all paper and board production, however, is extremely wasteful in terms of natural resource utilization as well as cost prohibitive in most instances and applications.
Various methods have been suggested in the past for improving the dry-strength and related properties of a sheet formed from fibrous materials such as paper or board materials formed of cellulose fiber. One alternative for improving the dry-strength properties of paper products, for example, involves the surface sizing of the sheet at a size press after its formation. While some of the critical properties of the product may be improved through sizing the surface of the sheet, many papermaking machines, for example, including board and newsprint machines, are not equipped with a size press. Moreover, only the properties of the surface of the sheet are appreciably improved through surface sizing. Surface sizing therefore is either not available to a large segment of the industry or is inadequate for purposes of improving the strength of the product throughout the sheet. The latter factor is especially significant since paper failures during printing, for example, are obviously disruptive to production and extremely costly.
A preferred alternative to surface sizing of a sheet is to increase the strength of the product through the addition of chemical additives directly to the fiber furnish prior to forming the sheet. Common additives at the wet-end of a paper machine, for example, include cationic starch or melamine resins. Among the problems presented by known wet-end additives used in the papermaking industry, however, are their relatively low degree of retention on the cellulose fiber during the initial formation of the sheet at the wet-end of the paper machine. In most applications, significant portions of the wet-end additives accompany the white water fraction as it drains through the wire due to high dilution and the extreme hydrodynamic forces created at the slice of a fourdrinier machine. Alternatively, a significant portion of the additive may be lost in solution during the dwell time between its addition to the stock and the subsequent formation of the sheet on the machine at prevailing operating temperatures. Accordingly, the potential benefits achievable through the use of known methods for internally strengthening fiber products have seldom been realized in practice. Indeed, when the cost of the chemical additives is additionally considered, any marginal benefits actually achieved have been largely disappointing.
The use of various natural and synthetic polymeric materials to improve the strength of the fiber to fiber bond and the water resistance has also been suggested. The use of a polymeric binder, particularly in larger amounts such as 10 weight percent up to 20 weight percent or more, provides a very difficult problem. Even if the polymeric binder is not substantially water soluble and therefore becomes incorporated in the paper web, during drying of the web the polymeric binder can become very sticky and stick to the felts and drying cans employed in commercial operations. As a result, the entire operation must be shut down due to the sticking problem. The potential benefits of using larger percentages of a polymeric binder in a paper based product are therefore lost as such products simply cannot be made from a practical point of view.
The application of various release agents to paper making drier surfaces as well as to heated platens in pressing glue-coated wood particles is well known for preventing the sticking of resin to such surfaces. However, such application of a surface lubricant means the addition of another process step with the consequent increase in production time.
In U.S. Pat. No. 5,034,097, a composition is described which comprises epoxidized polyamide wet-strength resin and lecithin. The addition of lecithin allegedly eliminates the sticking problem encountered on the heated dryers in the manufacture of paper, and particularly in the manufacture of molded pulp products. The lecithin is preferably dispersed in an emulsifying or dispersing agent prior to its incorporation in the epoxidized polyamide. The epoxidized polyamide containing lecithin is then added to the pulp slurry prior to forming the molded product or paper on the wire mesh. Alternatively, each of the epoxidized polyamide and lecithin can be added separately to the aqueous pulp slurry.
Despite the various attempts to overcome the sticking problem, however, the industry is still searching for a solution which can be effectively and most efficiently employed.
Accordingly, an object of the present invention is to provide a process for efficiently making a paper based product by employing a polymeric binder, and more specifically a polymeric latex binder.
Another objective of the present invention is to prepare such a paper based product using commercial papermaking equipment where the product can comprise 10 weight percent up to 20 weight percent and more of the polymeric binder.
Still another object of the present invention is to provide an efficient process for making a paper based product having excellent physical properties, especially tensile strength.
These and other objects of the present invention will become apparent upon a review of the following specification and the claims appended thereto.